The present disclosure relates to a method of controlling charging and discharging of a battery energy storage device, and particularly, to a method of controlling charging and discharging of a battery energy storage device by performing a droop control using state of charge (SOC) offset values and SOC feedback gain values and preventing overcharging or over-discharging that may occur due to long time driving in order to efficiently and stably operate an energy storage device to be driven for a long time, and the battery energy storage device for the same.
A battery energy storage system (BESS) performs functions of storing, in a battery, power generated in a grid or providing power stored in the battery to the grid or a load.
FIG. 1 illustrates a view for explaining a typical BESS.
Referring to FIG. 1, a BESS 1 is connected to a grid 2 and a load 3, and may store power provided from the grid 2 or, on the contrary, provide power to the grid 2 or the load 3.
On the other hand, when receiving a target power value from an energy management system (EMS), which is an upper layer controller, according to a control mode, the BESS 1 provides the target power value to the grid 2 or charges the battery. When a frequency or a voltage of the grid 2 is changed according to a sharp load or an incident, the BESS 1 provides active power or reactive power to the grid 2 by using a droop control.
The droop control is a method that an energy storage device controls active power or reactive power stored in the battery using slopes for a frequency variation and a voltage variation of the grid.
FIG. 2 is a block diagram for explaining the droop control in a typical energy storage device.
Referring to FIG. 2, the typical energy storage device obtains a difference between a reference frequency f0 and a frequency f of the grid and obtains a reference droop power Pdroop_ref by dividing the difference by a frequency droop coefficient. Then, the reference droop power Pdroop_ref is added to target power PDG0 of an energy management system and the added result is used as an output value of the energy storage device.
Similarly, the typical energy storage device obtains a difference between a reference voltage V0 and a voltage V of the grid, and obtains a reference droop power Qdroop_ref by dividing the difference by a voltage droop coefficient. Then, the reference droop power Qdroop_ref is added to target power QDG0 of the energy management system and the added result is used as an output value of the energy storage device.
Accordingly, the output value at this point becomes to have an output curve illustrated in FIGS. 3a and 3b. 
FIGS. 3a and 3b illustrate output curves of a frequency droop control and a voltage droop control in the typical BESS.
Referring to FIGS. 3a and 3b, when the load increases, the frequency decreases. Therefore, an energy device 10 performs charging for increasing an output. On the contrary, when the load decreases, the frequency increases. Therefore, the energy device 10 performs discharging for decreasing the output.
Basically, droop coefficients Rf and Rv used for a control may be derived by using the following Equation (1):
                                          R            f                    =                      -                                                            f                  max                                -                                  f                  min                                                            2                ⁢                                  (                                                            P                      max                                        *                                          η                      PCS                                                        )                                                                    ⁢                                  ⁢                              R            V                    =                      -                                                            V                  max                                -                                  V                  min                                                            2                *                                                                            S                      PCS                      2                                        -                                          P                      PCS                      2                                                                                                                              (        1        )            
However, since the typical droop controller performs charging or discharging without consideration of SOC of the battery, and since the system is stopped due to overcharging or over-discharging in a case of a long time control, driving of the energy storage device 10 may not be possible in an urgent situation.